The NST is configured as an off-axis Gregorian system consisting of a parabolic primary, prime focus field stop and heat reflector (heat-stop), elliptical secondary and diagonal flats. An off-axis design was chosen principally because of its vastly reduced stray light, since there is no central obscuration, which reduces the telescope’s MTF (Modulation Transfer Function) at high spatial frequency.
The primary mirror (PM) is 1.7 m with a clear aperture of 1.6 m with a final figure residual error of 16 nm rms. The PM is made of Zerodur from Schott. The PM was figured by Steward Observatory Mirror Lab. The off-axis nature of the PM presented a number of problems in measuring the figure of the mirror. The measurements of the figure used a Computer Generated Hologram (CGH) to change the wavefront of the interferometer to match the aspheric of the PM. Although this sounds simple in principal, it requiring nearly three years of effort to make it work and be reliable.
The focal ratio of the PM is f/2.4, and the final ratio is f/50. The 120" circular opening in the field (heat) stop defines an 80’‘x80" maximal square field of view (FOV) of the NST. The working wavelength range will cover 0.4 to 1.7 mm in the Coudé Laboratory one floor beneath the telescope, and all wavelengths including far infrared at the Nasmyth focus on an optical bench attached to the side of the telescope structure.
The NST is an open telescope structure with an equatorial mount. A key design issue for a large aperture solar telescope is the thermal control system to keep the temperature of the mirrors very close to, or below that of the ambient air. The dome employs a wind-gate and exhaust system to control the airflow from the wind, which helps to maintain the same temperature inside and outside the dome, and clear concentrations of heat in and around the optical paths. In addition, we have implemented a closed cycle, chilled air system as part of the OSS (Optical Support Structure) to limit so-called “mirror seeing” by sweeping away turbulent cells and directly cooling the PM. After each day of observations, the PM will be cooled overnight to ensure that it is somewhat cooler than ambient in the morning. The OSS and active support mirror cell were built and tested by DFM Engineering in Longmont, CO. The PM is supported by 36 actuators that can bend out low-order aberrations, such as those due to gravity and/or thermal effects. The OSS was installed and tested in BBSO in during the Fall of 2008.
Source: BBSO
